1. Field of the Invention
The present invention relates to a signal detector for an optical information reproducing apparatus in which intrinsic marks formed intermittently on an optical recording medium are used to reproduce a channel clock, and information is reproduced synchronously with the channel clock from the optical recording medium on which the information is recorded in the form of marks synchronously with the channel clock by edging each mark responsively to a 1 of record data that has been modulated so that N bits (N is larger than 0 and an integer) in a block serving as a unit of modulation represent 1's, and converted in an NRZI mode.
2. Description of the Related Art
One of techniques of formatting an optical disk designed for an optical disk unit is referred to as a sample servo technique. Sample servo is such that: an information track is divided into a servo field and a data field; a phase-locked loop (PLL) is configured in relation to specific pits, which are called clock pits, formed within the servo field in order to reproduce a channel clock; and information is recorded or reproduced synchronously with the channel clock.
The sample servo technique includes what is called a DBF technique. In the DBF, 4/11 modulation is employed as a method of modulating record data. Herein, one byte is converted into eleven channel bits so that four of the eleven channel bits represent 1's. During reproducing, binary coding can be performed by setting four bits, which signify the largest levels of a reproduced signal, to logical 1. The reproducing technique, which is referred to as difference detection, is characteristic of immunity to degradations in a reproduced signal. Thanks to the immunity to signal degradations, the difference detection has the advantage of being suitable for high-density recording.
A technique for improving recording density for an optical disk is referred to as mark edge recording. Herein, unlike mark position recording based on a principle that a mark position on a disk is associated with a 1, recording or reproducing is such that record data is reversed (NRZI conversion) every time a 1 is detected, and an edge of a mark is associated with a 1. Mark edge recording allows both edges of a mark to have meanings, thus yielding excellent recording efficiency and suiting for high-density recording. However, the aforesaid sample servo-based difference detection technique is supposed to be combined with the mark position recording technique. That is to say, difference detection cannot be effected in combination with mark edge recording, which hinders improvement of recording density.
An art attempting to solve this drawback by combining the sample servo technique with the mark edge recording technique has been disclosed in Japanese Patent Laid-Open No. H5-101396/1993. Herein, a record signal is recorded in the mark edge recording mode. Difference detection is then performed in order to extract four bits signifying the smallest differences between levels of a reproduced signal, which have been sampled synchronously with a channel clock during reproducing, and a mean level thereof. The four bits are regarded as associated with edges of marks and set to 1's.
In the procedure described in the Japanese Patent Laid-Open No. H5-101396/1993, a bit signifying a level of a reproduced signal that is close to a mean level is associated with an edge of a mark and set to logical 1.
However, since an optical disk is fundamentally of a binary recording type, as long as recording or reproducing is performed in an ideal manner, a mean level will not arise. A reproduced signal has a mean level only when an edge of a mark formed on a disk is not read ideally because of a degraded MTF of an optical system or an insufficient bandwidth of a reproducer. A level a reproduced signal has in the vicinity of a mark edge depends greatly on a spacing between marks (line density), an MTF of an optical system, and a bandwidth of a reproducer. These requirements are governed mainly by a difference of an optical system or a circuit in an apparatus from that in other apparatus, a position (inner or outer circumference) on a disk, a type of a disk (medium), and recording conditions. In short, a reproduced signal does not necessarily have a means level at an edge of a mark.
The art described in the Japanese Patent Laid-Open No. H5-101396/1993 poses a detrimental problem in terms of stability in recording or reproducing. It cannot be said that employment of mark edge recording enables high-density recording.
When mark edge recording is effected through light modulation, an edge of a mark formed changes positions depending on recording conditions and eventually brings about a variation in mark length. In mark edge recording in which an edge position is significant, the variation leads directly to a detection error. Correction must therefore be performed during mark recording or reproducing. As for a procedure of such correction, examples are described in "Jitter Characteristics of Mark Recording on MO Disks" (Journal of the Japanese Society of Applied Magnetism, Vol.16, No. 5, 1992) and Japanese Patent Laid-Open No. H2-183471/1990.
In these correction procedures, a change in position of an edge dependent on a pattern of write data itself is corrected during recording, and a variation in length thereof due to a temperature change is corrected during reproducing. In other words, during recording, preceding and succeeding data patterns are taken into consideration in order to control a light emission pattern of a laser. During reproducing, since only a spacing between a leading edge of a mark and a trailing edge thereof changes and a change in position of each edge is limited, leading and trailing edges are manipulated independently. For example, a procedure described in Japanese Patent Laid-Open No. H2-183471/1990 is such that data pulses detected responsively to leading and trailing edges of a mark are fed to synchronizers, independently binary-coded by the synchronizers, and then synthesized.
Even in the combination of difference detection and mark edge recording, when light modulation is employed above all, a change in position of a mark edge is unavoidable. A change in position of a mark edge breaks a positional relationship between a channel clock used for sampling and an edge. Even when difference detection is adopted, a detection error occurs readily.
Since a conventional procedure can be employed for correction during recording, a change in position of an edge dependent on a recording pattern can be suppressed relatively effortlessly. However, there is a difficulty in suppressing a variation in mark length due to a varying ambient temperature in the course of recording-time correction. Even when the difference detection technique is adopted, some correction must be performed during reproducing.
The difference detection technique features that an external clock is used independently of marks formed on a disk, a reproduced signal is sampled synchronously with the clock, and sampled levels are compared with each other (the positions of leading and trailing edges cannot be located until difference detection is performed on a final bit). Leading and trailing edges cannot therefore be distinguished from each other in advance. Unlike the prior art, a procedure of compensating for a variation in mark length by performing temporal correction after detecting leading and trailing edges independently cannot be employed as it is. A margin is diminished by a variation in mark length, which disables high-density recording.
For effecting high-density recording by making the most of an advantage of difference detection, it is understood that a channel clock (a clock used for sampling) must be reproduced correctly so as to coincide with data positions on a disk. Because of various factors including a change in data writing conditions (including a change in ambient temperature), a phase difference is liable to occur between a channel clock reproduced responsively to clock pits and actual data written on a disk.
In an effort to solve this problem, a procedure has been disclosed in, for example, Japanese Patent Laid-Open No. H5-242608/1993. Herein, a reference pattern such as a pattern of a maximum repetition frequency is written at the start of each sector during data writing, and a reproduced signal originating from the pattern is used to optimize the phase of a sampling clock used for difference detection.
However, the procedure of recording a reference pattern such as a maximum repetition frequency pattern at the start of each sector makes it necessary to ensure a field, in which the reference pattern is formed, at the start of each sector. This causes a user-specific field to diminish in area. Eventually, a problem that high-density recording cannot be achieved occurs in handling an entire disk.
In the above procedure, after a clock is phased with the start of a sector, phasing is not carried out within the sector. The clock fails to track a change in data position resulting from uneven sensitivity of a disk within the sector. The phase difference between the clock and data therefore gets larger, causing a read error. Otherwise, this error may become a bottleneck in achieving high-density recording.